A method of making a segmented core optical waveguide preform for making
fiber that is resistant to attenuation increases due to hydrogen and heat
aging. A first core region comprising a silica glass rod containing at
least a first dopant is inserted into a central opening of a second core
region comprising silica soot containing a second dopant. The first core
region and second core region are consolidated together to form a
segmented core region and cladding is deposited on the outer surface of
the segmented core region.

This application is related to U.S. provisional application No. 60/063,441
filed on Oct. 29, 1997.

Claims

I claim:

1. A method of making an optical waveguide preform having a core region
comprising the steps of:

providing a first core region comprising a glass rod, the first core region
containing at least a first dopant;

depositing silica soot containing a second dopant on a mandrel, removing
the mandrel to provide a soot blank having a central opening therethrough
to provide a second core region;

inserting the first core region into the central opening in the second core
region and consolidating the first core region and second core region
together in a furnace to provide the core region of the waveguide preform,
the core region having an outer surface; and

depositing cladding comprising silica soot on the outer surface of the core
region.

2. The method of claim 1, wherein the glass rod is a silica glass rod and
the at least first dopant decreases the refractive index of the silica
glass.

3. The method of claim 2, wherein the at least first dopant comprises
fluorine.

4. The method of claim 3, wherein the second dopant increases the
refractive index of the second core region.

5. The method of claim 4, wherein the second dopant comprises germania.

7. The method of claim 1, wherein the step of providing the first core
region further comprises the steps of:

inserting the glass rod into a glass tube containing the at least first
dopant to provide an assembly, inserting the assembly into a furnace,
heating the assembly, flowing a gas selected from the group consisting of
100% chlorine and chlorine mixed with a diluent gas into the first end of
the tube, between the tube and the rod, and to the second end of the tube,
and collapsing the tube onto the rod in the furnace.

8. The method of claim 7, wherein the glass tube is silica glass and the at
least first dopant decreases the refractive index of the silica glass.

9. The method of claim 8, wherein the first dopant comprises fluorine.

10. The method of claim 9, wherein the second dopant increases the
refractive index of the second core region.

11. The method of claim 10, wherein the second dopant comprises germania.

12. The method of claim 11, wherein the glass rod is a silica glass rod and
contains dopant to increase the refractive index of the silica glass.

13. The method of claim 12, wherein the dopant contained in the rod
comprises germania.

14. The method of claim 13, wherein the waveguide preform is an optical
waveguide fiber preform.

15. A method of making a dispersion modified optical waveguide preform
having a core region comprising the steps of:

inserting a silica glass rod containing a dopant to increase the refractive
index of the silica glass into a silica glass tube containing a dopant to
decrease the refractive index of the silica glass to provide an assembly,
inserting the assembly into a furnace, heating the assembly to a
temperature of at least about 1000.degree. C., flowing a gas selected from
the group consisting of 100% chlorine and chlorine mixed with a diluent
gas into the first end of the tube, between the tube and the rod, and to
the second end of the tube, and incrementally lowering the assembly into a
furnace zone at a temperature of at least about 1900.degree. C. to
collapse the tube onto the rod to provide a first core region;

depositing silica soot containing a dopant to increase the refractive index
of the silica on a mandrel, removing the mandrel to provide a soot blank
having a central opening therethrough to provide a second core region;

inserting the first core region into central opening in the second core
region and consolidating the first core region and second core region
together in a furnace to provide the core region of the waveguide preform,
the core region having an outer surface; and

depositing cladding comprising silica on the outer surface of the core
region.

16. The method of claim 15, wherein the dopant to increase the refractive
index of the silica glass rod and the dopant to increase the refractive
index of the silica soot comprises germania.

17. The method of claim 16, wherein the dopant to decrease the refractive
index of the silica glass tube comprises fluorine.

This invention relates to a method of making an optical waveguide preform.
More specifically, the method of the present invention is useful for
making low loss optical waveguides, especially waveguide fibers having a
segmented core profile.

Optical fibers having refractive index profiles such as W-profiles,
segmented core profiles, and the like possess desirable dispersion
characteristics. See U.S. Pat. Nos. 4,715,679 and 5,031,131 for teachings
of various kinds of dispersion modified optical fibers. Fibers having
these kinds of refractive index profiles have often been made by chemical
vapor deposition (CVD) processes such as plasma CVD processes that are
capable of forming single-mode fibers the cores of which include layers of
different refractive indices. Such processes produce relatively small
preforms. It is advantageous to form dispersion modified optical fiber
preforms by outside vapor deposition (OVD) processes which produce
relatively large preforms or draw blanks to decrease the cost of making
the fiber.

A typical OVD process for forming such fibers is disclosed in U.S. Pat. No.
4,629,485. In accordance with that patent, a germania-doped silica rod is
formed and stretched to decrease its diameter. A piece of the rod is used
as a mandrel upon which pure silica glass particles or soot is deposited.
The resultant composite structure is heated in a consolidation (drying and
sintering) furnace through which a fluorine-containing gas flows. The soot
is therefore doped with fluorine and sinters on the rod. One or more
additional layers of glass are formed on the outer surface of the
fluorine-doped silica layer to form a blank from which a fiber can be
drawn.

When soot is sintered in accordance with the aforementioned method, whereby
fluorine is supplied to the porous preform solely by way of the
fluorine-containing muffle gas, the fluorine concentration (as measured by
the .DELTA. of the fluorine-containing layer) is not sufficient to provide
certain desirable optical characteristics. The typical fluorine
concentration achieved with muffle gas doping provides a -0.4% .DELTA.
when CF.sub.4 is the fluorine-containing constituent. The maximum delta
value for CF.sub.4 produced by the above-described process is -0.5%
.DELTA..

As used herein, the term .DELTA..sub.a-b, the relative refractive index
difference between two materials with refractive indices n.sub.a and
n.sub.b, is defined as

.DELTA..sub.a-b =(n.sub.a.sup.2 -n.sub.b.sup.2)/(2n.sub.a.sup.2) (1)

For simplicity of expression, .DELTA. is often expressed in percent, i.e.
one hundred times .DELTA.. In this discussion, n.sub.a is the refractive
index of the fluorine-doped glass and n.sub.b is the refractive index of
silica.

When a fluorine-doped silica tube is collapsed onto a germania-doped silica
rod, or when a germania-doped silica tube is collapsed onto a
fluorine-doped silica rod, it is extremely difficult to achieve a
satisfactory interface between those two members. This is so because the
interface typically contains many seeds, and much of the resultant preform
or blank produces unusable optical waveguide. Such seed formation is less
prevalent when members formed of other glass compositions such as a
fluorine-doped silica tube and a pure silica rod are fused to form a
preform.

U.S. Pat. No. 4,675,040, discloses inserting a core glass rod made of pure
silica into a soot tube of cladding material made of pure silica doped
with fluorine and sintering the core/clad structure to fuse the cladding
over the pure silica core. U.S. Pat. No. 4,668,263 discloses a method for
collapsing a silica tube having a fluorine-doped inner layer onto the
surface of a silica rod. In accordance with that patent the collapse step
is accomplished by rotating the tube and heating it with the flame from a
longitudinally traveling burner. That technique could not be employed to
make dispersion modified fiber designs of the type that utilize the entire
fluorine-doped tube, including the outer surface, as part of the core
region or light propagating region of the fiber. The reason for this is
that, because the flame wets the glass, i.e. introduces hydroxyl
contamination, the resultant fiber would be rendered unsuitable for
operation at wavelengths where attenuation due to hydroxyl ions is large.
A further disadvantage of this method concerns the temperature of the
flame, which is not lower than 1900.degree. C. At such high temperatures,
control of the process becomes difficult. The axis of the preform can
become non-linear or bowed. If the core rod is a soft glass such as a
germania-doped glass, the rod can become softer than the tube; this can
result in an out-of-round core or a core that is not concentric with the
outer surface of the resultant fiber.

U.S. Pat. No. 4,846,867 discloses a method for collapsing a fluorine-doped
silica tube onto the surface of a silica rod. Prior to the tube collapse
step, a gas phase etchant is flowed through the gap between the rod and
tube while the tube is heated by a flame. In the specific examples,
wherein SF.sub.4 is the etchant, a gaseous mixture of SF.sub.6, Cl.sub.2
and oxygen (ratio 1:1:6 by volume) is introduced through a gap between the
rod and the tube. Such a gaseous mixture removes glass from the treated
surfaces of the rod and tube, thus forming new surfaces at the rod/tube
interface. The chlorine is present in an amount sufficient to remove water
generated by the fluorine-containing etchant. The outer surface of the
resultant preform is thereafter coated with silica soot particles that are
dried, doped with fluorine and then sintered to form a blank from which an
optical fiber is drawn. The flame that was directed onto the tube during
the gas phase etching step introduces water into the outer surface of the
tube. The attenuation of the fiber resulting from that water is high. The
attenuation at 1380 nm for one example is 30 dB/km which is attributed to
contact of the oxyhydrogen flame with the preform.

Copending U.S. patent application Ser. No. 08/795,687, filed on Feb. 5,
1997, entitled "Method of Making Optical Fiber Having Depressed Index Core
Region," discloses a method for inserting a germania doped silica glass
rod into a fluorine doped silica glass tube to form an assembly and
consolidating the assembly to form a seed free interface. The tube may be
overclad with cladding material such as pure silica. It has been
discovered that while this method avoids a seed free interface between the
consolidated germania-doped silica and fluorine-doped silica interface, it
is difficult to control attenuation increases due to hydrogen and heat
aging in fibers drawn from preforms made by this method. As used herein,
the term "hydrogen aging" refers an attenuation increase in an optical
waveguide that has been exposed to an atmosphere containing hydrogen at a
certain concentration, pressure and temperature. The term "heat aging"
refers to an attenuation increase exhibited by an optical waveguide that
has been exposed to heat.

In view of the disadvantages discussed above, it would be desirable to
provide a method for producing a segmented core optical waveguide preform
that allowed the entire light active region of waveguide preform blank to
be dried from the inside of the blank. In addition, it would be
particularly advantageous to provide a dispersion modified optical
waveguide that had low attenuation and exhibited minimal or no attenuation
increase due to heat or hydrogen aging.

SUMMARY OF THE INVENTION

The present invention relates to a method of making a an optical waveguide
preform having a segmented core region. The method comprises providing a
first core region comprising a glass rod, preferably a silica glass rod,
the first core region containing at least a first dopant, preferably a
dopant for decreasing the refractive index of the silica glass rod, such
as fluorine. The method further comprises depositing silica soot
containing a second dopant on a mandrel, removing the mandrel to provide a
soot blank having a central opening therethrough to provide a second core
region. The second dopant contained in the silica soot is preferably an
index increasing dopant such as germania. The method also comprises
inserting the first core region into the central opening of the second
core region together in a furnace to provide the segmented core region of
the waveguide preform. The method further includes the step of depositing
a cladding comprising silica soot on the outer surface of the segmented
core region of the waveguide preform.

In one embodiment of the invention, the step of providing the first core
region may further comprise inserting the glass rod into a silica glass
tube containing the first dopant to provide an assembly, inserting the
assembly into a furnace, heating the assembly, and collapsing the tube
onto the rod in the furnace. Preferably, a gas selected from the group
consisting of 100% chlorine and chlorine mixed with diluent gas is flowed
into the first end of the tube, between the tube and the rod, and to the
second end of the tube before the tube is collapsed onto the rod. The rod
is preferably a silica glass rod containing a dopant such as germania to
increase the refractive index of the glass.

The tube collapse step can be performed in the same furnace in which the
chlorine gas flowing step occurs. Advantageously, as the adjacent surfaces
of the rod and tube are cleaned by the gas while the assembly is in a
furnace, the outer surface of the tube is not contaminated by water that
would be present if a flame were employed for heating the assembly during
the gas flowing step. This method is especially suitable for forming an
optical fiber having a core that includes an annular region of depressed
refractive index, as disclosed in copending United States patent
application entitled, "Dispersion Managed Optical Waveguide Fiber," filed
on the same date as the present application, fibers having W-profiles, and
segmented core fibers.

A particular advantage of the method (of the present invention is that
preform that the entire light active region of a waveguide preform blank
can be dried from the inside of the blank. Applicant has discovered that
dispersion modified optical waveguide fibers made by the method of the
present invention exhibit minimal or no attenuation increase due to heat
or hydrogen aging. Additional features and advantages of the invention
will be set forth in the description which follows. It is to be understood
that both the foregoing general description and the following detailed
description are exemplary and explanatory and are intended to provide
further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary refractive index profile of an optical
fiber that may be produced by the method of this invention;

FIG. 2 illustrates an exemplary refractive index profile of an optical
fiber having a depressed index core region that may be produced by the
method of this invention;

FIG. 3 illustrates formation of a porous glass preform on a mandrel;

FIG. 4 illustrates sintering of a porous glass preform; and

FIG. 5 illustrates depositing soot onto the core region.

DETAILED DESCRIPTION

The method of this invention may be employed to produce an optical
waveguide preform having a segmented core index profile. Generally, this
method comprises (a) providing a first core region containing at least a
first dopant, (b) providing a second core region made by depositing silica
soot containing a second dopant on a mandrel and removing the mandrel to
provide a soot blank having a central opening therethrough, (c) inserting
the first core region into the opening in the second core region and
consolidating the first core region and second core region together in a
furnace to provide the segmented core region of the waveguide preform and
(d) depositing on the outer surface of the core region cladding comprising
silica soot. The core of the resultant fiber drawn from the waveguide
preform includes the inner core region and an outer core region, and each
core region optionally includes additional annular core regions. As used
herein, the term "core" refers to the light active area of the waveguide,
i.e. the region of the waveguide through which light is transmitted.

Steps (a) through (d) are not necessarily performed in the stated order. In
one embodiment, a silica glass rod containing a dopant to decrease the
refractive index of the glass is the first core region. In an alternative
embodiment, the step of providing the first core region 22 comprises
collapsing a glass tube 20 over a glass rod 21 (See FIG. 5) depositing
soot onto the core region, with the rod and tube assembly initially being
exposed to a chlorine gas at temperature sufficient to achieve cleaning of
the rod surface and inner surface of the tube. After the chlorine
cleaning, the temperature is then increased collapse and fuse the tube to
the rod in a dry furnace.

One embodiment of the invention involves producing an optical waveguide
fiber having a refractive index profile in which a central region of the
core has a lower refractive index than an annular region of the core
surrounding the central region. FIG. 1 shows an exemplary refractive index
profile of a waveguide fiber produced by the method of the present
invention. In this embodiment, the first core region 22 comprises a glass
rod, preferably a silica glass rod containing a dopant. The dopant
preferably decreases the refractive index of the silica glass. Fluorine is
the preferred dopant since attenuation due to B.sub.2 O.sub.3 limits fiber
usage to wavelengths less than about 1200 nm.

The first core region may be made by any suitable method for making a
waveguide core such as OVD, VAD, etc. For example, a fluorine doped rod
may be made by depositing a pure silica blank on a 0.25 inch or larger
alumina mandrel. The mandrel is removed to provide a central opening down
a central region of the blank, and the blank may be consolidated with
about 70 cc of CF.sub.4 to dope the blank with fluorine, 66 ccm (cubic
centimeters per minute) of chlorine and 1 liter of helium down the central
opening. The blank may then be incrementally exposed to a temperature of
at least about 1900.degree. C., preferably about 2050.degree. C. and
stretched to form a solid fluorine doped rod. The diameter of the rod will
depend on the desired index profile of the fiber that is formed from the
preform blank. For example, the fluorine doped silica glass rod may be
stretched to a diameter of about 8 mm.

As shown in FIG. 3, a second core region of the waveguide preform is formed
by depositing silica soot containing a second dopant on a relatively large
diameter mandrel 10 and removing the mandrel to provide a soot blank
having a central opening therethrough. Prior to the deposition step,
mandrel 10 is inserted through tubular handle 11. While mandrel 10
rotates, it also undergoes translational motion with respect to soot
generating burner 13, whereby a porous glass preform 12, which may serve
as the second core region, is built up on the mandrel. The mandrel 10 has
a large enough diameter to produce a tube structure having a sufficiently
large inner diameter to be useful in later steps of the method. For
example, an alumina mandrel having a diameter of 0.25 inches or larger is
sufficient. The mandrel may be in the form of a rod or rod or tube. U.S.
Pat. No. 5,180,410, the contents of which are relied upon and incorporated
by reference, includes a detailed description on forming porous preforms
on tubular mandrels, which may be useful for performing the step of
providing a tubular porous preform that may be doped during consolidation
in accordance with the method of the present invention.

As noted above, during deposition of the second core region, the mandrel
rotates and also undergoes translational motion with respect to a soot
generating burner to build up a soot preform on the mandrel. The second
dopant is preferably a dopant to increase the refractive index of silica,
such as germania. The amount of dopant in the second core region will
depend on the desired refractive index profile of the waveguide formed
from waveguide preform.

As shown in FIG. 4, after the mandrel 10 has been removed from second core
region 12 to provide a second core region having a central opening 18
therethrough, a handle 14 may be attached to one end of the second core
region 12 to allow the second core region soot blank to be suspended in a
consolidation furnace. Preferably, the handle 14 is a standard ball joint
handle 14 that is fused to handle 11, and the assembly including second
core region 12 is suspended in consolidation furnace 15 by that handle.
The first core region comprising the fluorine doped glass rod is inserted
into the central opening 18 through the second core region. The rod may be
suspended within the second core region soot blank any suitable method
such as making a standard small ball at one end of the rod and suspending
the ball inside the handle at the end of the second core region soot blank
(not shown). The first core region and second core region may be placed
together in a furnace at a temperature of about 1000.degree. C. to about
1100.degree. C., flowing helium at about 1 liter per minute and about 60
cubic centimeters per minute of chlorine between the central opening in
the second core region and the first core region for about 1 hour in the
direction of arrow 16. Muffle gas, preferably containing helium, is flowed
into the furnace as indicated by arrows 17. The end of the second core
region 12 may optionally contain a capillary tube 19. The first core
region and second core region are then consolidated together for about one
hour by incrementally lowering the first core region and second core
region assembly at a rate of a about 5 mm per minute into the zone 25 of a
furnace at a temperature of at least about 1400.degree. C., preferably
about 1500.degree. C, generated by a heater 24 to form the consolidated
core region 23.

After consolidation of the first and second core regions to provide the
segmented core region of the preform blank a standard ground joint handle
is fused to one end and cladding material comprising silica may be
deposited on the outer surface of the segmented core region of preform as
illustrated in FIG. 5. Before depositing cladding material, the segmented
core region may be heated to a temperature of at least about 2050.degree.
C., preferably about 2050.degree. C. and stretched to a diameter suitable
for the overcladding step.

In an alternative embodiment, the step of providing the first core region
may include additional steps. Dispersion modified fiber having a more
complicated refractive profile may require further processing steps to
achieve the more complicated index profile. An exemplary refractive index
profile for a dispersion compensating optical waveguide fiber is shown in
FIG. 2.

The index profile shown in FIG. 2 may be provided by the method of present
invention. In this embodiment, the step of providing first core region
includes inserting a silica glass rod into a silica glass tube containing
at least a first dopant to provide an assembly. Preferably, the silica
glass rod contains a dopant such as germania, P.sub.2 O.sub.5 or the like
to increase the refractive index of the silica glass rod. The rod can be
formed by any one of various known techniques such as modified chemical
vapor deposition (MCVD), vapor axial deposition (VAD) and outside vapor
deposition (OVD), depending upon its desired refractive index profile. The
at least first dopant contained in the tube is preferably a dopant such as
fluorine to decrease the refractive index of the tube. The tube/rod
assembly is inserted into a furnace at a temperature of about 1000.degree.
C. to about 1100.degree. C. Drying gas selected from the group consisting
of 100% chlorine and chlorine mixed with a diluent gas such as helium is
flowed through one end of the tube, between the tube and the rod and to
the second end of the tube for one hour to clean the outer surface of the
rod and inner surface of the tube.

The drying gas conventionally comprises a mixture of chlorine and an inert
gas such as helium. Although the flowing gas stream could contain a
diluent such as helium, 100% chlorine is preferred for cleaning purposes.
The gas streams consist of dry gases, whereby no water is present in the
vicinity of assembly during heat treatment. Gases can be purchased dry;
moreover, the helium used for the muffle gas is also run through a drier.

The diameter of the rod is advantageously slightly smaller than the inner
diameter of tube, allowing the chlorine to flow downwardly around the
entire periphery of the rod. The chlorine acts a hot chemical cleaning
agent. The chlorine cleaning step is more effective at high temperatures.
It is preferred that the temperature of the cleaning step be at least
about 1000.degree. C. to about 1500.degree. C., since at lower
temperatures, the duration of the step would be sufficiently long that the
step would be undesirable for commercial purposes. Obviously, lower
temperatures could be employed if processing time were not a concern. The
flow of hot chlorine between the tube and rod is very beneficial in that
it allows the surfaces of the two members to be brought together without
the formation of seeds at their interface. Seeds include defects such as
bubbles and impurities that can produce attenuation in the resultant
optical fiber.

After flowing the drying/cleaning gas for about an hour, one end of the
tube/rocd assembly is incrementally lowered into a zone of a furnace at
least about 1900.degree. C., preferably about 2050.degree. C., and the
tube is collapsed onto the rod and stretched to an appropriate diameter to
be inserted into the central opening through the second core region. The
top end of the rod may provided with an enlarged end which is suspended
from a narrow region at or near the handle of the tube. A vacuum source is
connected to the handle. The bottom tip of the tube/rod assembly is heated
in the zone of the furnace to a temperature of about 2050.degree. C. As
the tip of assembly passes through furnace zone, the diameter of the
assembly decreases, and the tube collapses onto the rod and the space
between those two members becomes evacuated. The assembly may be drawn
elongate the assembly into a first core region in which the tube is fused
to the rod.

The first core region may then be inserted into the central opening of the
second core region as described above with respect to the previous
embodiment. The remaining processing steps of providing a second core
region, inserting the first core region into a central opening through the
second core region, and providing a cladding layer comprising silica are
similar to the steps previously described in the embodiment described
above.

A fluorine-doped glass tube used in the step of making the first core
region may be made by inserting a mandrel through tubular handle. The
mandrel has a relatively large diameter in order to produce a tube having
a sufficiently large inner diameter to be useful for receiving the silica
glass rod. While the mandrel rotates, it also undergoes translational
motion with respect to a soot generating burner, whereby a porous glass
preform is built up on the mandrel.

The mandrel is removed from porous glass soot preform to provide a tubular
preform having a central opening therethrough. A standard ball joint
handle is fused to the tubular handle, and the preform is suspended in
consolidation furnace by the ball joint handle. Sintering is performed in
an atmosphere that includes a fluorine-containing centerline gas such as
SiF.sub.4, CF.sub.4, C.sub.2 F.sub.6, or the like. SiF.sub.4 tends to give
higher levels of fluorine doping (typically producing a -0.7% .DELTA. and
occasionally producing a delta of about -0.8%), but that dopant causes
elevated water levels in the resultant glass. Such elevated water levels
in the fluorine-containing glass can be tolerated if the fiber core has a
relatively high .DELTA.-value with respect to the silica cladding, whereby
little power propagates in the annular fluorine-containing region of the
fiber. CF.sub.4 results in dryer glass but does not give the high dopant
levels that can be obtained by using SiF.sub.4.

High concentrations of fluorine can be used in this process because porous
soot preform is formed of pure silica, i.e. there is no dopant such as
germania that could be disadvantageously diffused within the blank. The
resultant sintered tube contains a relatively high fluorine concentration
since fluorine-containing gas is flowed into the central opening in the
tube and outwardly through the pores of the porous glass preform whereby
it achieves maximum contact with the entire body of porous glass. The
muffle gas preferably contains a diluent gas such as helium and a
sufficient amount of chlorine to dry the preform.

The centerflow gas also preferably contains one or more diluent gases such
as helium and chlorine. The flow of chlorine can be discontinued after the
desired water content has been achieved and before the porous preform
sinters. The resultant fluorine-doped tube can be stretched or redrawn to
decrease the inside diameter to the desired size. If the tube is
stretched, it can then be cut to lengths suitable for the deposition of
soot thereon.

A boron-doped tube is simpler to make than a fluorine-doped tube. For
example, a porous SiO.sub.2 --B.sub.2 O.sub.3 preform could be formed on a
mandrel as described above with respect to the fluorine doped tube,
BCl.sub.3 being fed to the burner along with SiCl.sub.4. The mandrel is
removed, leaving a longitudinal central opening, and the preform is placed
into a consolidation furnace. A muffle gas of 40 standard liters per
minute (slpm) helium flows upwardly through the furnace muffle, and gases
of 1 slpm helium and 75 standard cubic centimeters per minute (sccm)
chlorine flows into the central opening. After the preform is dried, it is
sintered. The resultant tube can be stretched as described above.

A more detailed description of making porous preforms, forming core rods,
doping porous preforms and attaching handles to preforms may be found in
copending U.S. patent application Ser. No. 08/795,687, filed on Feb. 5,
1997, entitled "Method of Making Optical Fiber Having Depressed Index Core
Region," the contents of which are relied upon and incorporated by
reference.

Waveguide fibers produced by the method of the present invention exhibit
low attenuation as a result of the low seed count at the interface between
the first core region and the second core region. Attenuation at the water
peak of about 1380 nm for fibers made by the method of the present
invention is low since the tube is not heated by a flame. Fibers produced
by the method of this invention exhibit about 1 dB/km excess loss at the
water peak of about 1380 nm.

Fibers made by the method of the present invention also exhibit low heat
and hydrogen aging. Heat aging was measured by exposing fibers produced
according to the present invention to 200.degree. C. for 24 hours, and the
fibers exhibited an attenuation of less than about 0.02 dB/km. Hydrogen
aging was measured by exposing fibers made by the method of the present
invention to 85.degree. C. for 1 week at about 1% hydrogen, and the fibers
exhibited an attenuation of less than about 0.03 dB/km. Thus fibers
produced according to the present invention exhibit excellent resistance
to hydrogen and heat aging. An advantage to fibers produced according the
present invention is that the fibers do not require a hermetic coating to
prevent hydrogen and heat aging.

It will be apparent to those skilled in the art that various modifications
and variations can be made to the method of the present invention without
departing from the spirit or scope of the invention. Thus, it is intended
that the present invention cover the modifications and variations of this
invention provided they come within the scope of the appended claims and
their equivalents.